According to the state of the art, a range of possibilities exists for homogenising light. Thus for example the use of a refractive beam-forming element is known. A refractive element adapted to the surface shape hereby deflects proportions of the incident radiation in such a manner that the desired intensity distribution is produced in the homogenisation plane. It is problematic with devices of this type that a change in the intensity distribution at the input leads directly to a change in the intensity distribution at the output, i.e. in the homogenisation plane. As a result, the adjustment of devices of this type is critical, i.e. the installed position with respect to incident radiation directly influences the output distribution. In addition, even small impurities have great influence on the beam formation. Furthermore, devices of this type are suitable only for homogenisation of light bundles with a small diameter (less than 1 mm) and are complex in production.
Also the use of diffractive beam-forming elements or computer-generated holograms (CGH) are known for homogenisation of radiation. A bending phase element hereby bends power proportions of the incident radiation in such a manner that the desired beam profile is produced in the homogenisation plane. Such elements show great wavelength dependency as a result of the diffractive effect. Furthermore, the efficiency is dependent upon the number of height steps (discretisation) and the relief of the surface leads to an increased scattered light proportion.
In the context of the present invention, above all the so-called Fly's Eye condensers (FEC) are of interest. Such Fly's Eye condensers have, according to the state of the art, a regular microlens array (rMLA). The incident radiation impinges on this microlens array so that the lenses thereof focus the radiation. The maximum angle of the focused radiation thereby depends upon the numerical aperture (NA) of the lenses. Behind the focus of the lenses individual radiation bundles run divergently away from each other, the angle of divergence corresponding to the numerical aperture of the lenses. In the beam path behind the microlens array, a Fourier lens is now disposed, which deflects the individual bundles in such a manner that the partial beams produced by the individual microlenses of the microlens array are situated one above the other in the focal plane of the Fourier lens. Since all power proportions which run through individual lenses are superimposed on the same surface in the focal plane, the radiation is homogenised. The degree of homogenisation is thereby dependent upon the number of individual lenses. If the number of individual lenses is chosen to be sufficiently large, then the homogenisation is virtually independent of the input intensity distribution. However it is a condition that the numerical aperture of the incident radiation corresponds at most to that of the lenses. The extension of the homogeneously illuminated surface in the focus of the Fourier lens is determined by the numerical aperture of the lenses and the focal distance of the Fourier lens. The envelopes of the intensity distributions in the focal plane of the Fourier lens are the same when using an array of identical lenses (regular array) and an individual lens. When using an array of identical lenses, interference effects occur however in addition, said effects leading to a further intensity modulation with the occurrence of intensity maxima and zero positions of the intensity and hence to an impairment in homogeneity.
The spatial distance of the hereby occurring intensity maxima in particular is inversely proportional to the width of the individual lenses. This means that using smaller lenses leads to large spacings of the intensity maxima and hence to less homogeneity. On the other hand, also the use of the larger lenses however leads to an impaired homogeneity and to a greater dependency of the intensity distribution upon the input intensity distribution with an assumed constant width of the input intensity distribution.
In the previously mentioned technology, a modulation of the intensity distribution always occurs in the focus of the Fourier lens. This means that the intensity over the homogenisation surface is subjected to regular variations and drops away relatively gently at the edge. This modulation can be avoided if, parallel to the above-described first regular microlens array, an identical second regular microlens array is disposed. In the case of an individual lens, a very homogeneous distribution with a steep edge drop is consequently achieved. In the case of a large number of lenses (lens arrays), the above-described interference problems however occur in the same way.